1. Field of the Invention
The invention relates generally to a multiband RF receiver and a method for receiving a multiband RF signal, and more particularly to techniques of converting frequencies of the multiband RF signal.
2. Description of the Related Art
There is known a receiver for receiving through at least one antenna a multiband radio frequency (RF) signal having distinct RF bands.
FIGS. 1(a) and 1(b) illustrate two implementations of a conventional multiband receiver in functional block diagram, respectively.
Referring first to FIG. 1(a), a receiver 1 is illustrated in which two system bands (e.g., frequency bands licensed and allocated to carriers) are simultaneously sampled, as disclosed in, for example, Japanese Patent Application Publication No. 2001-274714.
In the receiver 1, radio frequency (RF) signals, upon reception at two antennas 101 and 101, enter bandpass filters (BPFs) 102 and 102 for respective system bands, to cancel out undesired frequency components.
Those two signals are next amplified by low noise amplifiers (LNAs) 103 and 103, respectively. The thus-amplified two signals are combined with each other at an adder into a composite signal, and this composite signal enters an analog-digital converter (A/D converter) 104.
The A/D converter 104 performs simultaneous frequency-conversion of the received signal by undersampling at a common sampling frequency Fs. The sampling frequency Fs is generated from a sampling-clock generator 105.
The sampling frequency Fs is selected by the direction of a processor (i.e., a CPU) 100 so as to successfully prevent interference between spectral replicas of all desired signals (e.g., channel signals) within each system band. The replicas are produced by the undersampling.
As a result, the A/D converter 104 outputs a low intermediate frequency (Low-IF) signal. In some cases, this Low-IF signal is demodulated by, for example, Fast Fourier Transform (FFT).
Referring next to FIG. 1(b), another implementation of a conventional receiver is illustrated within a dashed box.
In this implementation, two distinct RF bands, upon amplification at respective LNAs 103 and 103, enter respective mixers 112 and 112 to convert those two RF bands (i.e., native bands) into respective low-intermediate-frequency (Low-IF) bands.
Each mixer 112, 112 converts each native RF band to each Low-IF band whose frequency is lowered depending on each local oscillator frequency produced from each local oscillator 111, 111. The local oscillator frequencies for these two local oscillators 111 and 111 are determined, on a desired-signal by desired-signal basis, to be mutually different enough to prevent interference between desired signals (e.g., channel signals) within each system band.
These two IF band signals, after filtered, are combined with each other at an adder into a composite signal, and the composite signal having distinct IF bands is simultaneously oversampled by the A/D converter 104.
In this regard, the term “undersampling” is used to mean a process of sampling a received signal at a frequency lower than a Nyquist frequency, to thereby achieve intentional aliasing, ultimately for converting a high frequency carrier wave into a low frequency wave, as disclosed in, for example, “Direct Bandpass Sampling of Multiple Distinct RF Signals,” co-authored by Dennis M. Akos, Michael Stockmaster, James B. Y. Tsui, and Joe Caschera, IEEE Transactions on communications, Vol. 47, No. 7, July 1999, pp. 983-988. This document is incorporated herein by reference in its entirety.
This process, owing to its lower sampling frequency, can make a receiver available with a relatively low ability of processing, while requiring a noise reduction filter with high performance because of the need to eliminate effects of folding noise.
In contrast, the term “oversampling” is used to mean a process of sampling a received signal at a frequency higher than a Nyquist frequency. This process, owing to its higher sampling frequency, enables a filter for noise reduction to be designed with ease, while requiring a receiver with high performance.
There is also known an alternative technique of sampling simultaneously-received distinct RF signals by undersampling at a sampling frequency, and of defining a range of sampling frequencies that prevents interference between those RF signals, upon sampled, as disclosed in, for example, “Direct Downconversion of Multiband RF Signals Using Bandpass Sampling,” co-authored by Ching-Hsiang Tseng and Sun-Chung Chou, IEEE Transactions on Wireless Communications, Vol. 5, No. 1, January 2006, pp. 72-76. This document is incorporated herein by reference in its entirety.
More specifically, this technique is practiced in a manner that each actual case for calculating an acceptable sampling frequency is classified as one of pre-assumed groups, according to the frequency of folding frequency location resulting from the sampling of each RF signal, and that the acceptable sampling frequency is calculated from both the undersampling factor and the frequency of each RF signal.
Referring next to FIG. 2, a wave form chart is illustrated for explanation of frequency conversion from an RF signal to an IF signal, and subsequent frequency conversion from the IF signal to a Low-IF signal.
As illustrated in FIG. 2, first and second RF bands lie in a higher-frequency region.
First, these first and second RF bands are converted into first and second IF bands, as conceptually expressed with the following formula based on a single local oscillator frequency F0:FIF1=FR1−F0, andFIF2=FR2−F0,
where
FIF1: center frequency of first IF band,
FIF2: center frequency of second IF band,
FR1: center frequency of first RF band,
FR2: center frequency of second RF band, and
F0: single local oscillator frequency.
Next, these first and second IF bands are simultaneously undersampled at an undersampling frequency Fs. These first and second IF bands, as a result of repeated aliasing by the undersampling, are converted into Low-IF bands located not higher than Fs/2.